cmd/dist: implement fetch prototype

With the rename of fetch to fetch-object, we now introduce the `fetch`
command. It will fetch all of the resources required for an image into
the content store. We'll still need to follow this up with metadata
registration but this is a good start.

Signed-off-by: Stephen J Day <stephen.day@docker.com>

cmd/dist/main.go

@@ -61,6 +61,7 @@ distribution tool
 		},
 	}
 	app.Commands = []cli.Command{
+		fetchCommand,
 		fetchObjectCommand,
 		ingestCommand,
 		activeCommand,